Fracturing is a common stimulation method for increasing the production of hydrocarbons from oil, gas and geothermal wells. This method is particularly suitable in the production of fluids and natural gas from low permeability formations. Such reservoirs typically require multiple fractures to reach economic production levels and provide effective drainage. When multiple fractures are required, the casing in a zone of interest, after being perforated and stimulated, must be hydraulically isolated before any new zone of interest can be exploited. Isolation of zones often consists of inserting a mechanical plug, hereinafter referred to as a bridge plug, below the zone of interest. The bridge plug hydraulically isolates that portion of the well from a lower portion (or the rest) of the well. The isolation of the lower zone ensures that high pressure fracturing fluid pumped into the well is directed to the zone of interest. The high pressure fracturing fluid is used to fracture the formation at the open perforations in the casing. The high pressure of the fracturing fluid initiates and then propagates a fracture through the formation.
Wirelines are typically used to run a bridge plug into a vertical well. This method, however, is not effective in horizontal wellbores since treatment fluids (necessary for efficient production of hydrocarbons) are displaced by the bridge plug. While coiled tubing may be used to push and set the bridge plug into a horizontal wellbore, the use of such tubing is time consuming and expensive.
Construction of a sand plug in a horizontal wellbore has further been explored in order to hydraulically isolate a zone from the lower portion of the wellbore. In order to build a sand plug, it is necessary that the end of the fracturing fluid include a pill of fluid containing a greater amount of sand or proppant in comparison to the amount of sand or proppant present in the fracturing fluid. An objective is to have the sand or proppant remain suspended in the fluid pill. The fluid pill is pumped into the well under the fracturing pump rate. The pumping, and thus displacement of the fracturing fluid, is stopped as the fluid pill reaches the perforation tunnels at the zone of interest. During this time, the fluid pill, with high concentration of sand, remains stationary within the wellbore as the fractures are allowed to partially close. Once the fractures are partially closed, displacement of the fluid pill is resumed, normally at a low rate in comparison to the pump rate during the fracturing process. Typically, the rate is set low enough to prevent the fractures from reopening. The fluid pill moves into the perforation tunnels and into the fractures. The high concentration of sand or proppant suspended within the fluid pill screens out against the fractures which are partially closed. Subsequently, the suspended sand in the fluid pill bridges off against the fractures. As the process continues, the sand continues to pack off against the perforation tunnels and eventually the sand packs off against itself creating a sand plug in the wellbore. The slow rate of pumping is continued until the pressure within the wellbore rises indicating that a proper sand plug has been built within the wellbore.
Constructing a sand plug within a horizontal wellbore is difficult since gravitational settling of sand or proppant in the wellbore causes the plug to settle along the bottom of the wellbore which creates a void or channel along the upper side of the wellbore. Isolation of the treated zone is compromised since the displacement fluid passes down the channel and into the fracture without allowing the sand plug to form. Thus, it is necessary in sand plug construction that the fluid pill remain stationary for a time sufficient to keep the sand or proppant suspended in order that fractures in the formation will at least partially close.
U.S. Pat. No. 7,735,556, herein incorporated by reference, discloses a method which remedies the problems associated with insufficient suspended sand or proppant prior to closing of fractures in the formation. This method uses an ultra lightweight (ULW) proppant or neutrally buoyant proppant, in combination with sand, to build a sand plug. The method facilitates multizone fracturing treatments in horizontal wellbores not seen with conventional sand isolation plugs and provides improved isolation between perforated intervals. In the method, a fluid pill containing the ULW proppant or neutrally buoyant proppant is pumped into the wellbore at the end of the fracturing treatment. The fluid pill is pumped down the wellbore until it almost reaches fractures within the zone of interest. The pumping is then ceased or reduced, allowing the fractures to partially close. The ULW proppant remains suspended within the fluid pill while stationary. The pumping is then resumed at a slow rate as a short pump burst. This causes the proppant in the fluid pill to bridge off until a bridge plug is formed.
Methods have been sought for enhancing the wellbore isolation capabilities of the process disclosed in U.S. Pat. No. 7,735,556.